For me, I usually go dungeon to get stuff to sell. Items from dungeons are always sold at 100% profit, but most of the items aren't worth selling, such as Ingredients, which drops the most, since very few townsfolk would buy them unless they are on the Showcase at the window.

Townsfolk generally is enough to fund your business, but you can unlock playable characters as the story progresses, and as you move on to the newer dungeons. These playable characters will also be your customers, as you would have noticed Louie showing up in your shop a few times already.

In the case that you don't want to be spoilt in anything, I suggest progressing with dungeon, but make sure you are able to keep your debts paid on time.

Dungeons are closed on Sundays, so use that day to sell all your stuff. Keep an eye out for Booms, they are great ways to earn quick cash and customer reputation.

Puzzlemint's advice keeps you so far in the black that you can usually go dungeoning a few times a week at the very least, if not daily. But they're abysmal for profit -- even the bonus dungeon in the postgame nearly always gives less profit-per-time than just buying and selling the highest tier of merchant goods that you could unlock by week three.

The key thing to realize is that customers' wallets grow tremendously the first couple times you gain a rep level with them, and they only gain rep points if you make a sale without haggling (and three times as fast if you sell in the 104-107% area). After two levels -- the heart over their heads -- almost anyone coming to the store will be able to afford the highest tier of stock, and even little girls will be casually dropping five figures.

Yeah as everyone said above the dungeon is not the way to go if your primary interest is success in your shop. Pretty much just do it for fun and to get a little more story if you want to; otherwise it just wastes way too much time for little payoff, especially early on when you can hardly carry anything in the dungeons.

There's one reason to go to the second dungeon (i.e. the first "real" dungeon, after the training dungeon): At the end of it, if you beat the boss on 15F, there is a Very Odd Vase that's a guaranteed drop.

Basically Puzzlemint's advice. Also try to pay attention to the items that are 'popular' and shove those in your storefront window. Keep an eye on what each type of customer likes (little girls like sweets and books iirc). The better the customer loyalty, the more money they'll have to spend. I got by with only giving things a 10% markup at most, most of the time, unless the market price was up.

If you're going for new game plus, buy as much crap as you can because items carry over but money does not.